Compiler: fix type descendent for union metaclass (#12308)

spec/compiler/codegen/is_a_spec.cr

@@ -894,4 +894,15 @@ describe "Codegen: is_a?" do
       x.is_a?(Union(B, C))
     )).to_b.should be_false
   end
+
+  it "restricts union metaclass to metaclass (#12295)" do
+    run(%(
+      x = true ? Union(String | Int32) : String
+      if x.is_a?(String.class)
+        1
+      else
+        2
+      end
+    )).to_i.should eq(2)
+  end
 end

src/compiler/crystal/semantic/type_intersect.cr

@@ -100,6 +100,12 @@ module Crystal
     def self.common_descendent(type1 : GenericClassInstanceMetaclassType | GenericModuleInstanceMetaclassType, type2 : MetaclassType | VirtualMetaclassType)
       return type1 if type1.instance_type.generic_type.metaclass == type2
 
+      # Special case: a GenericInstanceMetaclass with a union type is
+      # the metaclass of a union type, for example Union(String | Int32).class.
+      # In that case the intersection of that type with a non-union metaclass
+      # will never match: it must also be another union type.
+      return nil if type1.instance_type.is_a?(UnionType)
+
       restricted = common_descendent(type1.instance_type, type2.instance_type)
       restricted ? type1 : nil
     end